set truecolor support as a default

[nekopsykose]

closes #1127

by default, tcell is already reading COLORTERM, hence why the non-disabled part of the code works without actually calling/doing anything.

this changes the default to supporting colorterm for users with COLORTERM, and allowing them to opt out with MICRO_TRUECOLOR=0 (or unsetting COLORTERM in their terminal, since that is a requirement of tcell).

this is the opposite of the previous behaviour, where the default is no-truecolor and it has to be opted-into as well as having COLORTERM support, but i think in the modern age it makes more sense to support 'true' colors as a default. nevertheless, it is a 'breaking' change for users.

the documentation has also been updated, but i notice now these two sections seem identical; perhaps one of them can be removed entirely, not sure.